The present invention relates to a perforated tray without downcomer, a perforated tray tower without downcomer, and a method of distillation, and in particular, relates to (1) a perforated tray without downcomer that enables stable distillation of an easily polymerizable compound, such as (meth)acrylic acid, and a liquid containing such an easily polymerizable compound (hereinafter, may be referred to collectively as xe2x80x9can easily polymerizable compoundxe2x80x9d) over an extended period of time while effectively inhibiting the formation of polymers, (2) a perforated tray tower without downcomer fitted with such perforated trays without downcomer, and (3) a method of distillation using the perforated tray tower without downcomer.
To manufacture commercial products, it is widely practiced in the manufacturing industry to distil and refine an easily polymerizable compound, such as (meth)acrylic acid or an ester thereof, in the presence of oxygen or a polymerization inhibitor for the purpose of preventing polymerization of the compound. Besides, it is well known that perforated trays without downcomer and a perforated tray tower without downcomer fitted with the trays are used for distillation.
Each perforated tray without downcomer disposed in such a perforated tray tower without downcomer has holes formed almost uniformly all over the tray, except for those holes for clamps, bolts, etc. that attach the tray to a support beam, a support ring, etc. Typically, the holes (perforations) are formed by a punching press or a drill. It is known that in a perforated tray without downcomer, specifications on, among other things, the hole diameter and the hole shape, the tray thickness, and the opening ratio, are very important factors in improving performance, as well as specifications on the tray-to-tray interval and the levelness when the perforated tray without downcomer is disposed in the perforated tray tower without downcomer (See, Properties of Perforated Trays Without Downcomer, Vol. 34, Chemical Engineering, page 567 to 571, 1970, by Hashimoto et al, and Distillation Engineering Handbook published by Asakura Shoten 1966).
However, in some cases, distillation of, for example, (meth)acrylic acid, using the conventional perforated tray tower without downcomer causes formation of a polymer which should be removed manually or chemically after suspension of the operation.
Accordingly, to solve the above problems, techniques, each realizing a specific state, are suggested as follows to further inhibit polymerization during distillation using the perforated tray tower without downcomer. Each technique is aimed at improving inhibition of polymerization by using a polymerization inhibitor (stabilizer agent).
In a first state, the inside of the perforated tray tower without downcomer is wet with a liquid containing an easily polymerizable compound. This is aimed at bringing a gaseous phase portion into contact with a polymerization inhibitor by bringing the gaseous phase portion into contact with a liquid phase portion, because the polymerization inhibitor is present primarily in the liquid phase portion (liquid portion) and less so in the gaseous phase portion (gas portion).
U.S. Pat. No. 3,717,553 discloses an example of a specific technique to effect the first state, whereby the holes of the perforated tray near the inner wall of the tower are formed with an extended diameter or is arranged to form a slit, so that the inner wall of the tower and the back surface of the perforated tray are completely wet with liquid.
U.S. Pat. No. 3,988,213 discloses a technique to prevent polymerization on the tower wall by effecting the opposite to the first state, whereby the temperature of the tower wall is kept higher than that of the gaseous phase portion by providing a jacket so as to prevent the gas including no polymerization inhibitor from condensing and polymerizing on the tower wall.
In a second state, gas and/or liquid currents are prevented from channelling and stagnating. This is aimed at preventing the polymerization inhibitor from being unevenly distributed or insufficiently dispersed due to channelling and stagnating currents.
Japanese Laid-Open Patent Application No. 10-212249/1998 (Tokukaihei 10-212249) and corresponding European Published Patent Application 0 856 343 A1 disclose an example of a specific technique to effect the second state, whereby a support ring (perforated tray support member) is punched to form a hole through which the liquid on the support ring flows down smoothly. Japanese Laid-Open Patent Application No. 10-76103/1998 (Tokukaihei 10-76103) discloses another example of a specific technique to effect the second state, whereby the perforated trays without downcomer are separated from the tower wall by a distance of 1 mm to 15 mm to prevent liquid currents from channelling and stagnating near the tower wall.
Each of these techniques is able to prevent the formation of polymers to some extent, but not sufficiently. The conventional techniques, although successfully effecting either the first state or the second state through an additional specified arrangement in the tower and perforated trays, fails to concurrently and surely effect both the states.
Another problem is that providing a jacket as above requires large investments in equipment and strict temperature controls, which is not very practical. Further referring to the jacket method above, the tower wall is not made wet (kept dry) so as to prevent the gas from condensing, thus effecting a state that is opposite to the first state.
Accordingly, there is a demand for a distillation technique to concurrently and surely effect both the first state and the second state, and eventually to more effectively prevent the formation of polymers and perform stable distillation over an extended period of time.
In view of the above problems, the present invention has an object to offer a perforated tray without downcomer, a perforated tray tower without downcomer, and a method of distillation that are capable of effectively preventing the formation of polymers and stably distil an easily polymerizable compound, such as (meth)acrylic acid, over an extended period of time, and that are especially suited for use in distillation of an easily polymerizable compound, such as (meth)acrylic acid.
The inventors of the present invention have diligently worked to achieve the object, and as a result, have found that both the first state and the second state can be surely effected so that a stable distillation can be carried out over an extended period of time with the formation of polymers being effectively prevented, if (i) the thickness and opening ratio of the perforated tray without downcomer, as well as the hole diameters and the hole pitches, are regulated in specific ranges, (ii) the intervals between, and the inclinations (levelness) of, the perforated trays without downcomer disposed in the tower, as well as the distance between the two most closely located holes when a plurality of perforated tray without downcomer is used at the same stage, are regulated in specific ranges, and (iii) the perforated tray tower without downcomer is used for distillation under specific conditions, which has led to the completion of the invention.
Specifically, in order to achieve the foregoing object, the perforated tray without downcomer in accordance with the present invention satisfies following conditions (a) to (d):
(a) The hole diameter, d, is in a range of from 10 mm to 25 mm.
(b) The center-to-center distance between two adjacent holes is in a range of from 1.2 d to 3 d.
(c) The tray thickness is in a range of from 2 mm to 8 mm.
(d) The opening ratio is in a range of from 10% to 30%.
Further, the perforated tray tower without downcomer in accordance with the present invention is fitted with two or more such perforated trays without downcomer and satisfies following conditions:
(g) The interval between the perforated trays without downcomer is in a range of from 0.1 D to 0.5 D, where D is the tower diameter.
(h) The levelness of the perforated tray does not exceed 8 mm.
(i) The two most closely located holes that belong respectively to adjacent perforated trays without downcomer are separated by a distance ranging from 50 mm to 150 mm when a plurality of perforated trays without downcomer are used at the same stage.
Further, the method of distillation in accordance with the present invention is a method to distil an easily polymerizable compound or a liquid containing an easily polymerizable compound, using the perforated trays without downcomer.
Distillation of an easily polymerizable compound by means of specified perforated trays without downcomer or perforated tray tower without downcomer satisfying the foregoing conditions have at least following advantages:
(1) The gas and liquid flow uniformly on the perforated tray without downcomer with a smaller gradient in liquid concentration and a more uniform dispersion in concentration of polymerization inhibitors.
(2) The gas and liquid flow smoothly without channelling, and stagnate for a relatively short period of time.
(3) The perforated tray without downcomer and the inner walls of the tower are wet with a liquid containing a polymerization inhibitor. The inside of the tower is therefore almost completely wet with no parts being left dry.
(4) The liquid does not stagnate on the perforated tray without downcomer.
According to the present invention and its advantages described above, both the first state and the second state are concurrently and surely effected with the formation of polymers being effectively prevented.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.